The present invention relates to a control method of an exhaust gas purifying system to remove NOx (nitrogen oxides) in an exhaust gas of an internal combustion engine through the reduction using an NOx occlusion reduction type catalyst. More particularly, it relates to a control method of an exhaust gas purifying system to restore the state of the NOx occlusion reduction type catalyst deteriorated through a sulfur poisoning.
Various studies and proposals have been made relating to an NOx catalyst for removing NOx by reducing it from an exhaust gas of an internal combustion engine such as diesel engine, kind of gasoline engines and various combustion equipments.
Among them, there is an exhaust gas purifying system for an internal combustion engine wherein an NOx occlusion reduction type catalyst is arranged in the exhaust passage of an engine, as described in Laid-Open Japanese Patent Publication 2000-274279. In the exhaust gas purifying system, NOx is absorbed in the NOx occlusion reduction type catalyst when the air/fuel ratio of an inflowing exhaust gas is lean. Then, when the NOx absorption capacity gets close to its saturation point, the control for regenerating the catalyst is done. By this control, the oxygen concentration of the inflowing exhaust gas is lowered by setting its air/fuel ratio to the theoretical air/fuel ratio or to rich condition for discharging the absorbed Nox, and the discharged NOx is reduced by using an annexed precious catalytic metal.
This NOx occlusion reduction type catalyst supports an NOx occluding substance (NOx absorbing material) made of an alkaline-earth metal such as Barium (Ba) and a precious catalytic metal such as platinum (Pt) on a catalyst support. Then, under a high oxygen concentration atmosphere, NO in the exhaust gas is oxidized by the catalytic activity of the platinum to change into NO2. This NO2 diffuses in the catalyst with a form of NO3—, and absorbed by the NOx occluding substance with a form of nitrate.
When the air/fuel ratio becomes rich and the oxygen concentration lowers, NO3— will be discharged from the NOx occluding substance with a form of NO2. This NO2 is reduced to N2, under the catalyst activity of the platinum by reducers such as unburned HC, CO and H2 contained in the exhaust gas. This reduction effect permits to prevent NOx from being discharged in the atmosphere.
However, since a sulfur content contained in the fuel of the diesel engine is accumulated in the NOx occluding substance to stabilize as sulfate, the NOx occlusion reduction type catalyst has a problem of sulfur poisoning that the amount of NOx occlusion reduces.
If the deterioration of NOx occluding substance progresses by the sulfur poisoning, the NOx purifying efficiency declines because the NOx absorbing capacity is lowered, even if the air/fuel ratio of the exhaust gas is in lean state and the oxygen concentration is high. In addition, because the NOx absorbing activity lowers immediately to the proximity of its limit, a frequent regeneration processing by a rich-burn is required. As a result, the fuel efficiency deteriorates.
Consequently, a regeneration processing is required to restore the NOx absorbing capacity of the NOx occluding substance by releasing under the low oxygen concentration atmosphere NOx absorbed under the high oxygen concentration atmosphere through changing over from the lean state to the rich state. In addition, a catalyst deterioration restoring process to remove the sulfur content using sulfur purge is also required at a phase when the deterioration has progressed to some extent by monitoring the progress of the catalyst deterioration caused by sulfur poisoning.
In the sulfur purge, however, it is necessary to raise the catalyst temperature as high as around 600° C. to 700° C. and to keep a reducing atmosphere as well. Therefore, a temperature and a rich state of the exhaust gas is needed for the sulfur purge different from that of the exhaust gas during the regeneration process to restore the NOx absorbing capacity of the NOx occluding substance.
Besides, it is necessary to raise the exhaust gas temperature to 600° C. or more for the sulfur purge. However, a lot of fuel is required to make the catalyst temperature around 600° C. to 700° C. for the sulfur purge by performing the controls such as intake throttling and post-injection to raise the exhaust gas temperature from the low temperature of the lean state of normal operation condition. And the fuel consumption deteriorates. As a result, it is particularly important to make the rich state optimal for the sulfur purge.